Field of the Invention
The present invention relates to a control system for a compressor motor used with an air-conditioning unit or a heat pump, or more in particular to a control system for controlling the electric current flowing in the windings of the motor to heat the motor and preheat the compressor. Generally, an air-conditioning unit or a heat pump comprises, as disclosed in U.S. Pat. No. 3,466,888, a refregeration system including a refrigerant compressor, an indoor coil (indoor heat exchanger), an outdoor coil (outdoor heat exchanger), a reversal valve and a restrictor on the one hand, and a control system for a compressor motor for driving the compressor, an indoor fan for supplying indoor air to the indoor coil, an outdoor fan for supplying outdoor air to the outdoor coil and a solenoid for driving the reversal valve on the other hand.
The heat pump acquires heat energy from the outdoor air through the outdoor coil, and applies the particular heat energy to the indoor air through the indoor coil to heat the indoor air. In the case where the heat pump is started when the compressor is low in temperature at low outdoor temperature, the heat energy acquired from the outdoor air is consumed to heat the compressor and therefore fails to heat the indoor air sufficiently. It thus takes a long time before the indoor air is sufficiently heated.
If the heat pump is in operation for a long time at a low outdoor temperature, on the other hand, the surface of the outdoor coil is frosted to reduce the heat exchange efficiency of the outdoor coil. Therefore, the surface of the outdoor coil must be defrosted before the frost forms in a great amount. The outdoor coil is defrosted by reversing the flow of the refrigerant gas passing through the outdoor coil by the reversal valve as an example. The outdoor fan is normally kept stopped during the defrosting operation. The heat energy required for defrosting is mainly derived from the heat energy accumulated in the compressor as a whole.
Japanese Patent Unexamined Publication No. 157970/82 discloses a system for heating a compressor by the current flowing in an electric heater arranged around the compressor. This system applies current to the electric heater to heat the compressor before starting a heat pump. As a result, the time required for heating the indoor air sufficiently is reduced. The heater of this system which is arranged on the surface of a compressor case, however, is difficult of heating the interior of the compressor. Further, since heat easily escapes outside of the compressor case, the efficiency of heating the compressor is low. Also, the compressor is naturally high in cost.